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Algebraic Number Theory — Problem Sheet #3

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Algebraic Number Theory — Problem Sheet #3 Algebraic Number Theory — problem sheet #3 1. (i) Show that AQ = R × (Zb ⊗ Q). (ii) Use Theorem 5.4 to show that for any extension L=K of number fields AL ' AK ⊗K L. (iii) Let L=K be a finite extension of number fields, Show that the trace maps trLw=Kv : Lw ! Kv define a continuous additive homomorphism trL=K : AL ! AK , whose restriction to L is the trace map L ! K. (iv) Repeat (iii) for the norm map and the idele group. 2. (i) Let F be a local field. If f 2 S(F ) and g(x) = f(ax + b), compute gb in terms of fb. n (ii) Let F=Qp be finite. Compute the Fourier transform of the characteristic function of b+πF oF . Deduce that the Fourier transform maps S(F ) to itself, and that the Fourier inversion formula holds for S(F). (iii) Verify that if F = R or C then ζ(f; s) = ΓF (s), where ( 2 e−πx if F = f(x) = R −2πxx¯ e if F = C 3. Let F=Qp be finite, and f 2 S(F ). s −s (i) Show that if f(0) = 0, then ζ(f; s) 2 C[q ; q ], hence is analytic for all s 2 C. (ii) Show that in general ζ(f; s) is analytic except for a simple pole at s = 0 with residue f(0)= log q. If you like analysis, investigate the analyticity of ζ(f; s) for an arbitrary f 2 S(F ) when F is archimedean. 4. Let L=K be an extension of number fields of degree n. Suppose that almost all places v of K split n completely in L (i.e. #fwjvg = n). By comparing ζL(s) and ζK (s) show that L = K. P 5. Let K be a number field, and S ⊂ ΣK a finite set of places. Let m = v2S mv · (v) be a finite formal linear combination of places of K with positive coefficients mv. Define 8 F ∗ v complex, or v2 = S real > v > ∗;+ ∗ <Fv = R>0 v 2 S real Uv;m = 1 + πmv O∗ v 2 S finite > v v > ∗ :Ov otherwise Q and set UK;m = v Uv;m (i) Show that UK;m is an open subgroup of JK , and that every open subgroup of JK contains some UK;m. ∗ (ii) Show that Clm(K) = JK =K UK;m is finite. (It is called the ray class group modulo m.) ∗ Q mp (iii) Show that if K = Q and 1 2 S then Clm(Q) ' (Z=NZ) , where N = p2S p . (iv)∗ Let Im(K) be the free abelian group on the finite places of K not in S, viewed as a subgroup of ∗ the group of fractional ideals of K. Let Pm(K) be the group of principal ideals xoK , where x 2 K satisfies: (a) for all finite v 2 S, v(x − 1) ≥ mv; and (b) for all real v 2 S, x is positive under the embedding K,−! R determined by v. ∼ Show that there is a natural isomorphism θ : Clm(K) −! Im(K)=Pm(K) — more precisely, that there is a unique isomorphism θ with the property that for every finite w2 = S, if x = (xv) is the idele with −1 xw = πw and xv = 1 for all v 6= w, then θ(x) is the class of the fractional ideal Pw . [The −1 is a convenient normalisation. It is not the case that for every idele x, θ(x) is the class of the −1 fractional ideal cS(x) . Frohlich¨ used to call this the “fundamental mistake of class field theory”.] 1 ∗ ∗ ∗ 6. (Characters) (i) Let F=Qp be finite. A character of F is a continuous homomorphism χ: F ! C . ∗ ∗ Show that the kernel of any character is an open subgroup of F . (Hint: it’s enough to show that ker χ\oF ∗ is open. First show that χ(oF ) ⊂ U(1); then observe that the open subset V = fz j jargzj < π=2g does not contain any non-trivial subgroup of U(1).) ∗ ∗ (ii) We say χ: F ! C is unramified if χ(oF ) = 1. Show that a character is unramified if and only if it s is of the form χ(x) = jxj for some s 2 C. (iii) [∗ from here on, increasing in number.] An idele class character or Hecke character of a number ∗ ∗ field K is a continuous homomorphism χ: JK =K ! C . Show that for any Hecke character χ, there ∗ ∗ exist continuous χv : Kv ! C (v 2 ΣK ) such that (a) for almost all finite v, χv is unramified; ∗ Q (b) if x 2 K , then v χv(x) = 1; Q (c) if x = (xv) 2 JK then χ(x) = v χv(xv). Conversely, show that for any family of characters (χv)v satisfying (a-b), there is a unique Hecke char- acter χ such that (c) holds. (iii) Show that if for each infinite place v, χv(x) 2 {±1g, then χ factors through some ray class group Clm(K), and in particular has finite order. 0 (iv) A Hecke character is said to be algebraic if there are integers mv, nv, nv such that: mv ∗ ∗ (a) if v is real, χv(x) = x for all positive x 2 Kv = R ; 0 nv nv ∗ ∗ (b) if v is complex, χv(z) = z z¯ for every z 2 Kv ' C . Show that if K is totally real (i.e. has no complex places) and χ is an algebraic Hecke character, then m = m is independent of v, and that χ(x) = φ(x) jxjm for some Hecke character φ of finite order. v A ∗ Q d (Hint: show that there exists d ≥ 1 such that if " 2 oK is a unit, vj1 χv(" ) = 1. Then use the unit theorem.) p x (v) Let K = Q( −7) ⊂ C. Write w for the unique place over 7, and if x 2 Ow, let 7 denote the Legendre symbol of (x mod πw) 2 kw = F7. πv Show that if v 2 ΣK;f − fwg then there is a unique πv 2 oK with Pv = (πv) and 7 = 1. Show that there is a unique Hecke character χ of K such that • for all finite v 6= w, χv is unramified and χv(πv) = πv; ∗ x • for all x 2 Ow, χw(x) = 7 . 0 Show that χ is algebraic, and that (nv; nv) = (−1; 0) for the complex place v of K. 1Many people call these quasi-characters, and reserve the term “character” for a continuous homomorphism χ: F ∗ ! U(1).
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